i) Field of the Invention
The present invention relates to an LED (light emitting diode) printer for sensitizing a photosensitive body surface by the light emitting operation of an LED array for use in a facsimile machine, a printer or the like, and a driving control method of its LED array in the printer.
ii) Description of the Related Arts
Conventionally, an LED printer is known. The LED printer is a printer provided with an LED array and a photosensitive body. The LED array is driven by an LED array chip (driver IC) on the basis of externally supplied data. This data represents a print pattern to be formed on the surface of the photosensitive body. A plurality of LED elements constituting the LED array selectively emit light according to this data. The emission duty cycle of each LED element is determined by, for example, a strobe signal. When the LED array emits light in this manner, the surface of the photosensitive body is sensitized. Hence, according to the data supplied to the LED array, the image pattern to be formed on the surface of the photosensitive body is determined.
The photosensitive body usually has a drum form. This photosensitive drum is rotated at a predetermined peripheral velocity. With the rotation of the photosensitive drum, the photosensitive portion formed on the surface of the photosensitive drum is moved along the peripheral direction of the photosensitive drum.
Further, the LED array is composed of a plurality of LED elements arranged in parallel. With the rotation of the photosensitive drum, the LED array is arranged in parallel along the direction (the longitudinal direction of the photosensitive drum) perpendicular to the moving direction of the photosensitive drum surface (the peripheral direction of the photosensitive drum). This arranging direction, that is, the longitudinal direction of the photosensitive drum is referred to as a main scanning direction and the moving direction of the photosensitive body surface (the peripheral direction of the photosensitive drum) is referred to as a sub-scanning direction.
When one line is printed by the LED printer, while the data representing the image pattern along the main scanning direction are given to the driver IC, the photo-sensitive body surface is moved in the sub-scanning direction (that is, the rotary drum is rotated). At this time, since the LED elements constituting the LED array continuously emit the light for a predetermined time, conventionally, the energy distribution along the subscanning direction on the photosensitive body surface becomes a convex form, and therefore the dot appearance is bad.
Further, with the rotation of the photosensitive drum, since the photosensitive body surface is moved relative to the LED array, in general, the sensitized area of the photosensitive body surface by the LED element of the single dot (hereinafter referred to as a light emission area per one dot of the LED element) is fixed. When characters, symbols or the like are printed, smoothness of an outline is limited by the light emission area per one dot (see FIGS. 10 and 14). Since the light emission area per one dot is fixed, for improving the smoothness of the outline by the LED printer, a certain fixed limitation is present.